The long-term goal of this program is to elucidate the genetic and functional correlates of drug transporters that influence inter-individual variations in drug disposition and therapeutic outcome. Variations in the 170 kd membrane bound, efflux transporter P-glycoprotein (P-gp, often referred to as multidrug resistance protein-1 or MDR-1) in adult subjects produce up to a 7-fold difference in bioavailability of digoxin, a P-gp substrate. P-gp is expressed in tissues central to in vivo drug disposition, including the liver, gut, blood-brain barrier, placenta, and kidney. While a number of genetic variants of MDR-1 have been demonstrated in continuous cell-lines, their frequency and clinical significance has not been fully established. A more robust and reliable RT-PCR method was developed recently to simultaneously quantitate MDR-1 mRNA and isolate 3.8 kb cDNA from total cellular RNA. As a result, an efficient cloning and expression of MDR-1 cDNA in an expression vector is now feasible. We will use these cDNA vectors to systematically evaluate functional variation of a large series of cDNA from leukemia patients. With this proposal, we will identify genetic variations that produce significant effects on the efflux function of Pgp with the following aims: Aim 1: to identify genetic variants of MDR- 1 from DNA and RNA isolated from leukemic (AML and MDS) patients. Aim 2: to determine functional significance of genetic variants identified from the leukemic patients By integrating the results of Aims 1 and 2, we can begin to define the role of genetic variations on the P-gp efflux function. The proposed studies will elucidate the role of MDR-1 genetic polymorphism in functional P-gp variations and may shed light on their modulation of systemic and CNS availability and disposition of drugs from a wide range of therapeutic classes.